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plone.dexterity 2.1

Dexterity

Dexterity is a system for building content types, both through-the-web and
as filesystem code. It is aimed at Plone, although this package should work
with plain Zope + CMF systems.

Key use cases

Dexterity wants to make some things really easy. These are:

Create a “real” content type entirely through-the-web without having to
know programming.

As a business user, create a schema using visual or through-the-web tools,
and augment it with adapters, event handlers, and other Python code
written on the filesystem by a Python programmer.

Create content types in filesystem code quickly and easily, without losing
the ability to customise any aspect of the type and its operation later
if required.

Support general “behaviours” that can be enabled on a custom type in a
declarative fashion. Behaviours can be things like title-to-id naming,
support for locking or versioning, or sets of standard metadata with
associated UI elements.

Easily package up and distribute content types defined through-the-web,
on the filesystem, or using a combination of the two.

Philosophy

Dexterity is designed with a specific philosophy in mind. This can be
summarised as follows:

Reuse over reinvention

As far as possible, Dexterity should reuse components and technologies
that already exist. More importantly, however, Dexterity should reuse
concepts that exist elsewhere. It should be easy to learn Dexterity
by analogy, and to work with Dexterity types using familiar APIs and
techniques.

Small over big

Mega-frameworks be damned. Dexterity consists of a number of specialised
packages, each of which is independently tested and reusable. Furthermore,
packages should have as few dependencies as possible, and should declare
their dependencies explicitly. This helps keep the design clean and the
code manageable.

Natural interaction over excessive generality

The Dexterity design was driven by several use cases (see docs/Design.txt)
that express the way in which we want people to work with Dexterity. The
end goal is to make it easy to get started, but also easy to progress from
an initial prototype to a complex set of types and associated behaviours
through step-wise learning and natural interaction patterns. Dexterity
aims to consider its users - be they business analysts, light integrators
or Python developers, and be they new or experienced - and cater to them
explicitly with obvious, well-documented, natural interaction patterns.

Real code over generated code

Generated code is difficult to understand and difficult to debug when it
doesn’t work as expected. There is rarely, if ever, any reason to scribble
methods or ‘exec’ strings of Python code.

Zope 3 over Zope 2

Although Dexterity does not pretend to work with non-CMF systems, as
many components as possible should work with plain Zope 3, and even where
there are dependencies on Zope 2, CMF or Plone, they should - as far as
is practical - follow Zope 3 techniques and best practices. Many
operations (e.g. managing objects in a folder, creating new objects
or manipulating objects through a defined schema) are better designed in
Zope 3 than they were in Zope 2.

Zope concepts over new paradigms

We want Dexterity to be “Zope-ish” (and really, “Zope 3-ish”). Zope is a
mature, well-designed (well, mostly) and battle tested platform. We do
not want to invent brand new paradigms and techniques if we can help it.

Automated testing over wishful thinking

“Everything” should be covered by automated tests. Dexterity necessarily
has a lot of moving parts. Untested moving parts tend to come lose and
fall on people’s heads. Nobody likes that.

What’s it all about?

With the waffle out of the way, let’s look in a bit more detail about what
makes up a “content type” in the Dexterity system.

The model

The Dexterity “model” describes a type’s schemata (most types will have
only one) and metadata associated with those schemata. A schema is just
a series of fields that can be used to render add/edit forms and
introspect an object of the given type. The metadata storage is extensible
via the component architecture. Typical forms of metadata include UI
hints such as specifying the type of widget to use when rendering a
particular field, and per-field security settings.

The model is typically described in XML, though at runtime it is an
instance of an object providing the IModel interface from
plone.supermodel. Schemata in the model are interfaces with zope.schema
fields.

The model can exist purely as data in the ZODB if a type is created
through-the-web. Alternatively, it can be loaded from a file. The XML
representation is intended to be human-readable and self-documenting.
It is also designed with tools like ArchGenXML and Genesis in mind,
that can generate models from a visual representation.

The schema

All content types have at least one (unnamed) schema. A schema is
simply an Interface with zope.schema fields. The schema can be specified
in Python code (in which case it is simply referenced by name), or it
can be loaded from an XML model.

The unnamed schema is also known as the IContentType schema, in that the
schema interface will provide the Zope 3 IContentType interface. This
means that if you call queryContentType() on a Dexterity content object,
you should get back its unnamed schema, and that schema should be
provided by the object that was queried. Thus, the object will directly
support the attributes promised by the schema. This makes Dexterity
content objects “Pythonic” and easy to work with.

The class

Of course, all content objects are instances of a particular class.
It is easy to provide your own class, and Dexterity has convenient
base classes for you to use. However, many types will not need a class
at all. Instead, they will use the standard Dexterity “Item” and
“Container” classes.

Dexterity’s content factory will initialise an object of one of these
classes with the fields in the type’s content schema, and will ensure
that objects provide the relevant interfaces, including the schema
interface itself.

The preferred way to add behaviour and logic to Dexterity content objects
is via adapters. In this case, you will probably want a filesystem
version of the schema interface (this can still be loaded from XML if you
wish, but it will have an interface with a real module path) that you
can register components against.

The factory

Dexterity content is constructed using a standard Zope 3 IFactory
named utility. By convention the factory utility has the same name as the
portal_type of the content type.

When a Dexterity FTI (Factory Type Information, see below) is created,
an appropriate factory will be registered as a local utility unless one
with that name already exists.

The default factory is capable of initialising a generic ‘Item’ or
‘Container’ object to exhibit a content type schema and have the
security and other aspects specified in the type’s model. You can use
this if you wish, or provide your own factory.

Views

Dexterity will by default create an add view (registered as a local
utility, since it needs to take the portal_type of the content type into
account when determining what fields to render) and an edit view (
registered as a generic, global view, which inspects the context’s
portal_type at runtime) for each type. There is also a default main
view, which simply outputs the fields set on the context.

To register new views, you will normally need a filesystem schema
interface. You can then register views for this interface as you
normally would.

If you need to override the default add view, create a view for IAdding
with a name corresponding to the portal_type of the content type.
This will prevent Dexterity from registering a local view with the same
name when the FTI is created.

The Factory Type Information (FTI)

The FTI holds various information about the content type. Many operations
performed by the Dexterity framework begin by looking up the type’s
FTI to find out some information about the type.

The FTI is an object stored in portal_types in the ZMI. Most settings can
be changed through the web. See the IDexterityFTI interface for more
information.

When a Dexterity FTI is created, an event handler will create a few
local components, including the factory utility and add view for the
new type. The FTI itself is also registered as a named utility, to
make it easy to look up using syntax like:

getUtility(IDexterityFTI, name=portal_type)

The FTI is also fully importable and exportable using GenericSetup.
Thus, the easiest way to create and distribute a content type is to
create a new FTI, set some properties (including a valid XML model,
which can be entered TTW if there is no file or schema interface to use),
and export it as a GenericSetup extension profile.

Behaviours

Behaviors are a way write make re-usable bits of functionality that can
be toggled on or off on a per-type basis. Examples may include common
metadata, or common functionality such as locking, tagging or ratings.

Behaviors are implemented using the plone.behavior package. See its
documentation for more details about how to write your own behaviors.

In Dexterity, behaviors can “inject” fields into the standard add and edit
forms, and may provide marker interfaces for newly created objects. See
the example.dexterity package for an example of a behavior that provides
form fields.

In use, a behavior is essentially just an adapter that only appears to be
registered if the behavior is enabled in the FTI of the object being
adapted. Thus, if you have a behavior described by my.package.IMyBehavior,
you’ll typically interact with this behavior by doing:

my_behavior = IMyBehavior(context, None)
if my_behavior is not None:
...

The enabled behaviors for a given type are kept in the FTI, as a
list of dotted interface names.

The Dexterity Ecosystem

The Dexterity system comprises a number of packages, most of which are
independently re-usable. In addition, Dexterity uses many components from
Zope and CMF.

The most important packages are:

plone.alterego (Python)

Support for dynamic modules that create objects on the fly. Dexterity
uses this to dynamically create “real” interfaces for types that exist
only through-the-web. This allows these types to have a proper
IContentType schema, and allows local adapters to be registered for
this interface (e.g. a custom view with a template defined through the
web).

Note that if a type uses a filesystem interface (whether written manually
or loaded from an XML model), this module is not used.

plone.supermodel (Zope 3)

Supports parsing and serialisation of interfaces from/to XML. The XML
format is based directly on the interfaces that describe zope.schema type
fields, and is thus easily extensible to new field types. This has the
added benefit that the interface documentation in the zope.schema package
applies to the XML format as well.

Supermodel is extensible via adapters and XML namespaces. plone.dexterity
uses this to allow security and UI hints to be embedded as metadata in the
XML model.

plone.behavior (Zope 3)

Supports “conditional” adapters. A product author can write and register
a generic behaviour that works via a simple adapter. The adapter will
appear to be registered for types that have the named behaviour
available.

Dexterity wires this up in such a way that the list of enabled behaviours
is stored as a property in the FTI. This makes it easy to add/remove
behaviours through the web, or using GenericSetup at install time.

plone.folder (CMF)

This is an implementation of an ordered, BTree-backed folder, with Zope 3
dictionary-style semantics for managing content items inside the folder.
The standard Dexterity ‘Container’ type uses plone.folder as its base.

Adds convention-over-configuration support for plone.supermodel schemata
and plone.autoform form hints.

plone.dexterity (CMF)

This package defines the FTI and content classes, provides basic views
(with forms based on z3c.form), handles security and so on. It also
provides components to orchestrate the various functionality provided
by the packages above in order to bring the Dexterity system together.

plone.directives.dexterity (CMF)

Adds convention-over-configuration support for Dexterity content and
add/edit forms.

plone.app.dexterity (Plone)

This package contains all Plone-specific aspects of Dexterity, including
Ploneish UI components, behaviours and defaults.

Usage examples

Take a look at the example.dexterity package, which can be found in the
Plone Collective (http://dev.plone.org/collective), for examples of various
ways to use Dexterity.

Changelog

2.1 (2013-01-01)

Added Finnish translations.
[pingviini]

Overrride allowedContentTypes and invokeFactory from PortalFolder
to mimic the behavior of Archetypes based folders. This allows the
registration of IConstrainTypes adapters to actually have the
expected effect.
[gaudenzius]

The default attribute accessor now also looks through subtypes
(behaviors) to find a field default.
[malthe]

Added support in the FTI to look up behaviors by utility name when
getting additional schemata (i.e. fields provided by behaviors).

This functionality makes it possible to create a behavior where the
interface is dynamically generated.
[malthe]

For now, no longer ensure that Dexterity content provides ILocation (in
particular, that it has a __parent__ pointer), since that causes problems
when exporting in Zope 2.10.
[davisagli]

Don’t assume the cancel and actions buttons are always present in the
default forms.
[optilude]

1.0a3 - 2010-01-08

require zope.filerepresentation>=3.6.0 for IRawReadFile
[csenger]

1.0a2 - 2009-10-12

Added support for zope.size.interfaces.ISized. An adapter to this interface
may be used to specify the file size that is reported in WebDAV operations
or used for Plone’s folder listings. This requires that the sizeForSorting()
method is implemented to return a tuple (‘bytes’, numBytes), where numBytes
is the size in bytes.
[optilude]

Added support for WebDAV. This is primarily implemented by adapting content
objects to the IRawReadFile and IRawWriteFile interfaces from the
zope.filerepresentation package. The default is to use plone.rfc822 to
construct an RFC(2)822 style message containing all fields. One or more
fields may be marked with the IPrimaryField interface from that package,
in which case they will be sent in the body of the message.

In addition, the creation of new files (PUT requests to a null resource) is
delegated to an IFileFactory adapter, whilst the creation of new directories
(MKCOL requests) is delegated to an IDirectoryFactory adapter. See
zope.filerepresentation for details, and filerepresentation.py for the
default implementation.
[optilude]